Suppose we mold a hollow piece of plastic into the shape of a double concave len
ID: 3894845 • Letter: S
Question
Suppose we mold a hollow piece of plastic into the shape of a double concave lens. The "lens" is watertight, and its interior is filled with air.
Does the lens converge or diverge the beam of light?
-- the answer is converge...BUT i thought convex lenses were converging light! Concave lenses diverge light they make small far images seem close and bigger like a rear view mirror in your car. Does doubling it... like reverse it or something or is my original thinking wrong to begin with. I do not have my textbook to get conceptual info :(
an explanation would be amazing! (especially cuz i have other problems of this type)
Explanation / Answer
The ability of a lens to act as converging or diverging depends upon its refractive index. The refractive index n of the material of a lens depends upon the medium in which the lens is placed. Generally the lens is in air so n gives refractive index of material of lens with respect to air. If you place the convex lens in a medium other than air, then due to change in n, the focal length f of lens changes. if n decreases, f increases.
1/f=((refractive index of lens/refractive index of medium)-1)(1/R1-1/R2)
So if you immerse the concave lens in a liquid whose refractive index is greater than refractive index of material of lens(here it is hollow lens filled with air) then it will change its nature and become convexlens and thus it will start converging the rays rather than focusing them on a single point. Its true the other way round also that is convex also becomes concave.